The name Tu Bishvat (בשבט ט״ו ‎) is derived from the Hebrew date of the holiday, which occurs on the fifteenth day of Shevat.
"Tu" stands for the Hebrew letters Tet (ט)and Vav (ו), which together have the numerical value of 9 and 6, adding up to 15. Tu Bishvat is a relatively recent name; the date was originally "Hamisha Asar BiShvat"
(בשבט עשר-חמשה), also meaning "Fifteenth of Shevat".